Field of the Invention
The present invention relates to an image forming apparatus configured to form an image on a sheet.
Description of the Related Art
An image forming apparatus using a method of fixing a toner image transferred onto a sheet in a heating-type fixing device is known. In the above-mentioned image forming apparatus, however, a temperature of the sheet rises due to the heating during the fixing process. The rise in temperature of the sheet to a temperature equal to or greater than a given temperature may cause firm adhesion between toner particles or between toner and the sheet when the sheet is stacked in a sheet delivery tray, causing so-called “adhesion of delivered sheets,” which becomes a cause of an image defect. Therefore, before being stacked in the sheet delivery tray, the sheet having the toner image fixed thereon is required to be cooled by using an air-cooling fan, to cool the sheet to a temperature that does not cause the adhesion of delivered sheets.
In recent years, there has been a tendency to decrease a melting point of toner. Therefore, the temperature that causes the adhesion of delivered sheets is decreased. Further, a conveyance speed is increased because of improvement in image formation process speed. Under the influence of the facts described above, it becomes difficult to cool the sheet to the temperature that does not cause the adhesion of delivered sheets. As a result, the image defect caused by the adhesion of delivered sheets further hinders improvement in image formation speed. For example, in Japanese Patent Application Laid-Open No. 2016-53609, there is disclosed a configuration in which a delivered sheet cooling unit varies an airflow rate of a cooling fan, which is configured to take in outside air, depending on a paper sheet size.
With the related art method of taking in and cooling the outside air by the cooling fan, it may be sometimes difficult to suppress the occurrence of the adhesion of delivered sheets even with an increase in airflow rate. For example, under a high-temperature environment at an outside-air temperature exceeding 30° C., or when an attempt is made to enhance productivity by increasing an operation speed of the image forming apparatus, it becomes difficult to suppress the occurrence of the adhesion of delivered sheets. Further, not only the sheet after the fixing but also a heat-generating portion, such as an electrical component is required to be cooled in some cases.